"Plasma-assisted" or "plasma-enhanced" processing has many applications in semiconductor device fabrication. Plasma-enhanced processing is a technique in which a substantially ionized gas generates active, metastable neutral and ionic species that chemically or physically react to deposit thin material layers on, or to etch material layers from, a semiconductor substrate in a reactor processing chamber.
Advanced semiconductor VLSI technologies employ plasma processing for a number of important steps in device fabrication. For example, plasma processing permits lower processing temperatures and higher deposition rates for growth and deposition of thin layers of insulators, semiconductors, or metals. In addition, reactive ion etching (RIE) processes in low-pressure plasmas are used for anisotropic patterning of the submicron features in VLSI device structures.
Plasma-enhanced processing may use remotely-generated or locally-generated plasmas. A remote plasma is a plasma that is generated external to the reactor's semiconductor processing chamber. The plasma is guided into the processing chamber through a conduit from a remote plasma source, which is separated from the processing chamber where the plasma interacts with the semiconductor wafer for the desired fabrication process. An in-situ or localized plasma is a plasma that is generated within the semiconductor processing chamber where it interacts with the semiconductor wafer.
Conventional plasma processing equipment for etch and deposition applications usually employ a 13.56 MHz power source, a 2.5 GHz microwave source, or a combination of these energy sources for generating a plasma (glow discharge) from plasma feed gas. In typical systems, a plasma-generating radio-frequency power source connects electrically to an electrically conducting wafer holding device known as a wafer chuck. A radio-frequency energy source causes the chuck and wafer to produce a locally-generated radio-frequency plasma in the processing chamber with the semiconductor wafer. These systems typically include a showerhead assembly for injecting plasma-generating feed gas into the processing chamber. This is known as a parallel-plate configuration, due to the parallel surfaces of the chuck and showerhead. Still other configurations use a combination of local and remote plasmas.
In all of these known configurations, constraints exist which limit plasma process flexibility and capabilities. In localized plasma enhanced chemical vapor deposition (PECVD), for example, parent gas molecules are dissociated into precursor atoms and radicals which can deposit on substrates. The plasma supplies energy to break chemical bonds in the parent molecules that would only be broken by thermal decomposition if the plasma were not present. Parent molecule dissociation is accomplished in the plasma through collisions with electrons, ions, photons, and excited neutral species. Unfortunately, the precursor species are also subject to the same active environment which dissociated the parent molecules. This can lead to further dissociation or reaction of gas phase species to form more complicated radicals before the radicals can condense on the substrate. There is thus a wide spectrum of precursor species incident on the growing film.
A further complication of localized PECVD is that the substrate is immersed in the plasma region. This results in a large flux of charged species incident on the substrate during film deposition. This can lead to ion implantation, energetic neutral embedment, sputtering, and associated damage.
In addition, localized PECVD tends to deposit film in a very directional manner. This limits step coverage and conformality, resulting in thicker films in certain areas and thinner films in others, particularly at the bottom and along the bottom portions of trenches and contact vias. Thus, there are three major problems associated with conventional in-situ PECVD: adequate control over incident gas phase species, ion damage as a result of the substrate being immersed in the plasma region, and limited step coverage and conformality.
The use of a remote plasma system for remote plasma enhanced chemical vapor deposition (RPECVD) can help to alleviate all of these problems, but raises additional limitations relating to the transfer of active plasma species from the remote source to the semiconductor processing chamber. The lifetime of metastable oxygen, for example, typically allows pathlengths of 1-2 meters in the transfer conduit from a remote plasma source to the semiconductor processing chamber. The pathlength of a typical metastable excited noble gas species, e.g., He*, is only 5-30 cm. Nitrogen and various other activated species important to plasma-enhanced processing have similarly short, or even shorter, pathlengths. Therefore, an important limitation of remote plasma reactors is that desired activated species often cannot reach the semiconductor substrate in a sufficiently activated state and adequate concentration for efficient plasma-enhanced processing.
What is needed is a new and improved plasma-enhanced apparatus and method which overcomes the problems in the prior art techniques, including inadequacy of the control over incident gas phase species, ion damage to the substrate, limited step coverage/conformality, and the limited lifetime and diffusion length of the active species generated by remote plasma sources.